Current state of the art requires that both anaerobic digestion and aerobic composting processes be controlled continuously from start to finish with minimal air emissions. Additionally, anaerobic digestion is frequently followed by aerobic composting in order to convert the digestate into a commercially valuable compost or soil product. Air emissions are more significant during feedstock receiving, feedstock preparation, digestate handling, digestate land spreading, and compost pile turning (both teardown and rebuild), especially after the first stage of composting. If the process is fully enclosed through digestion and through the first stage of composting, emissions diminish significantly and are considered minimal, although not eliminated completely. Consequently, the anaerobic process and material handling operations cause the greatest emissions. Existing anaerobic systems today range from outdoor systems with no emission control to those with complete enclosure and exhaust treatment devices. The regulatory trend in North America is to regulate anaerobic digestion for the control of methane, odor, and hydrogen sulfide, and aerobic composting emissions for the control of odor, ammonia, volatile organic compounds (VOC), and/or greenhouse gases. This trend is causing an increase in the number of modified systems that utilize high solids anaerobic digestion and aerated static pile composting technologies.
Anaerobic digestion has been used in a wide range of applications on a global basis. There are millions of single-family microdigesters operating in Asia. There are approximately 100,000 low solids wastewater digesters and hundreds of medium solids manure and food processing byproduct digesters in service today. Over the past twenty five years there has been a steady increase in the development of high solids anaerobic digestion in Western and Northern Europe. Most recently this development has proven that stackable high solids organic waste from urban areas (food, paper, landscape, wood, etc.) can be digested effectively. These systems maintain temperature, physical pile structure, and moisture to facilitate fermentation and biomethane production. Aerated static pile composting was developed in 1973 by the USDA. Generally, aerated static pile composting involves a controlled aeration method, such as a piping system under the pile or piles, and a residence time of at least 14 days. Both digestion and composting generally involve grinding and then mixing the organic feedstock materials so each organic particle is relatively small. Low and medium solids systems require approximately ½ inch or less in its maximum dimension. Stackable high solids systems and composting commonly require approximately 6 inch or less in its maximum dimension. However, the cost of grinding is expensive and relatively slow so organic waste materials typically accumulate in an unprocessed and odorous state if the mass rate of incoming material is greater than the grinding rate. The cost and time requirement for grinding rises dramatically as the particle size requirement becomes smaller.
There are a number of underground digesters that are intended to minimize cost and produce usable biogas. The use of lined and covered lagoons as well as cast in place concrete plug flow systems are examples of underground digesters. However, the ability to digest un-ground high solids urban waste cost-effectively has not been possible with underground systems. There are a number of aerated static pile composting systems being practiced today to improve odor control. The use of membranes, tarps, or covers is increasing in the industry, to help limit fugitive emissions and improve moisture control. However, all of these systems still require grinding and pile turning. Because of the denseness of the feedstock material, pile depths are generally limited to between 4 feet deep and 17 feet deep.